On Fri, Jan 09, 2026 at 10:06:29AM +0100, Konrad Dybcio wrote:
> On 1/9/26 12:29 AM, Nathan Chancellor wrote:
> > An (admittedly problematic) optimization change in LLVM 20 [1] turns
> > known division by zero into the equivalent of __builtin_unreachable(),
> > which invokes undefined behavior if it is encountered in a control flow
> > graph, destroying code generation. When compile testing for x86_64,
> > objtool flags an instance of this optimization triggering in
> > msm_dp_ctrl_config_msa(), inlined into msm_dp_ctrl_on_stream():
> >
> > drivers/gpu/drm/msm/msm.o: warning: objtool: msm_dp_ctrl_on_stream():
> > unexpected end of section .text.msm_dp_ctrl_on_stream
> >
> > The zero division happens if the else branch in the first if statement
> > in msm_dp_ctrl_config_msa() is taken because pixel_div is initialized to
> > zero and it is not possible for LLVM to eliminate the else branch since
> > rate is still not known after inlining into msm_dp_ctrl_on_stream().
> > Change pixel_div to one to make the division well defined in the
> > presence of an unsupported rate, relying on the DRM_ERROR print to
> > indicate the error to the user.
> >
> > Fixes: c943b4948b58 ("drm/msm/dp: add displayPort driver support")
> > Link:
> > https://github.com/llvm/llvm-project/commit/37932643abab699e8bb1def08b7eb4eae7ff1448
> > [1]
> > Reported-by: kernel test robot <[email protected]>
> > Closes:
> > https://lore.kernel.org/oe-kbuild-all/[email protected]/
> > Signed-off-by: Nathan Chancellor <[email protected]>
> > ---
> > drivers/gpu/drm/msm/dp/dp_ctrl.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > index cbcc7c2f0ffc..e4731c059ed8 100644
> > --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> > @@ -2395,7 +2395,7 @@ static void msm_dp_ctrl_config_msa(struct
> > msm_dp_ctrl_private *ctrl,
> > bool is_ycbcr_420)
> > {
> > u32 pixel_m, pixel_n;
> > - u32 mvid, nvid, pixel_div = 0, dispcc_input_rate;
> > + u32 mvid, nvid, pixel_div = 1, dispcc_input_rate;
> > u32 const nvid_fixed = DP_LINK_CONSTANT_N_VALUE;
> > u32 const link_rate_hbr2 = 540000;
> > u32 const link_rate_hbr3 = 810000;
> >
> > ---
> > base-commit: 66691e272e40c91305f1704695e0cb340cd162ff
> > change-id: 20260108-drm-msm-dp_ctrl-avoid-zero-div-be5dc40cbc18
>
> Dmitry, would it be beneficial to throw an actual error when the rate is
> is mangled? i.e.
>
> diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> index aa2303d0e148..4f710b8e6bc6 100644
> --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> @@ -2404,9 +2404,9 @@ static int msm_dp_ctrl_link_retrain(struct
> msm_dp_ctrl_private *ctrl)
> return msm_dp_ctrl_setup_main_link(ctrl, &training_step);
> }
>
> -static void msm_dp_ctrl_config_msa(struct msm_dp_ctrl_private *ctrl,
> - u32 rate, u32 stream_rate_khz,
> - bool is_ycbcr_420)
> +static int msm_dp_ctrl_config_msa(struct msm_dp_ctrl_private *ctrl,
> + u32 rate, u32 stream_rate_khz,
> + bool is_ycbcr_420)
> {
> u32 pixel_m, pixel_n;
> u32 mvid, nvid, pixel_div = 0, dispcc_input_rate;
> @@ -2415,14 +2415,21 @@ static void msm_dp_ctrl_config_msa(struct
> msm_dp_ctrl_private *ctrl,
> u32 const link_rate_hbr3 = 810000;
> unsigned long den, num;
>
> - if (rate == link_rate_hbr3)
> + switch (rate) {
> + case link_rate_hbr3:
> pixel_div = 6;
> - else if (rate == 162000 || rate == 270000)
> - pixel_div = 2;
> - else if (rate == link_rate_hbr2)
> + break;
> + case link_rate_hbr2:
> pixel_div = 4;
> - else
> + break;
> + case 270000:
> + case 162000:
> + pixel_div = 2;
> + break;
> + default:
> DRM_ERROR("Invalid pixel mux divider\n");
> + return -EINVAL;
Well... In the ideal world, we can't end up here, PHY's
configure_dp_clocks (or qcom_edp_set_vco_div()) will fail if the link
rate is is invalid here. I think, we should return an error here, but
there is no need to propagate it further.
See the discussion at
https://lore.kernel.org/dri-devel/[email protected]/
> + }
>
> dispcc_input_rate = (rate * 10) / pixel_div;
>
> @@ -2458,6 +2465,8 @@ static void msm_dp_ctrl_config_msa(struct
> msm_dp_ctrl_private *ctrl,
> drm_dbg_dp(ctrl->drm_dev, "mvid=0x%x, nvid=0x%x\n", mvid, nvid);
> msm_dp_write_link(ctrl, REG_DP_SOFTWARE_MVID, mvid);
> msm_dp_write_link(ctrl, REG_DP_SOFTWARE_NVID, nvid);
> +
> + return 0;
> }
>
> int msm_dp_ctrl_on_stream(struct msm_dp_ctrl *msm_dp_ctrl, bool
> force_link_train)
> @@ -2525,10 +2534,11 @@ int msm_dp_ctrl_on_stream(struct msm_dp_ctrl
> *msm_dp_ctrl, bool force_link_train
>
> msm_dp_ctrl_configure_source_params(ctrl);
>
> - msm_dp_ctrl_config_msa(ctrl,
> - ctrl->link->link_params.rate,
> - pixel_rate_orig,
> - ctrl->panel->msm_dp_mode.out_fmt_is_yuv_420);
> + ret = msm_dp_ctrl_config_msa(ctrl, ctrl->link->link_params.rate,
> + pixel_rate_orig,
> +
> ctrl->panel->msm_dp_mode.out_fmt_is_yuv_420);
> + if (ret)
> + return ret;
>
> msm_dp_panel_clear_dsc_dto(ctrl->panel);
>
>
>
> Konrad
--
With best wishes
Dmitry
